Fan with collapsible blades, redundant fan system, and related method

ABSTRACT

A redundant fan system for a computer includes two fans installed in series with at least one of the fans having collapsible blades. A fan system of this type reduces the fan inefficiency caused when one fan in a series mounted pair is not operating, either because it is free-wheeling or in a locked rotor condition. When non-operational, the fan blades of the collapsible fan fold inward due to airflow generated by the operational fan over the collapsible blades. The ability of the blades to fold reduces the inefficiency of the operational fan, having less of an effect on fan life. Also, because the flow of air is less restricted, proper airflow can be maintained, thus preventing overheating of the computer.

FIELD OF THE INVENTION

The present disclosure relates to fans having collapsible blades andredundant airflow systems that employ fans with collapsible blades. Thepresent disclosure further relates to systems that can provide redundantairflow in a variety of applications, including computers, electroniccommunications, appliances and automotive applications, while reducingthe problem of fan inefficiencies that are present when anon-operational fan is in an inoperative locked rotor or free-wheelingcondition.

BACKGROUND INFORMATION

Fans are critical components of computers, providing cooling airflow toprocessors, microchips, and other components. When fans fail, internaltemperatures rise, which at the least necessitates shutting down thecomputer and replacing the failed fan. At the worst, the fan failuregoes unnoticed by support personnel, causing improper performance oreven permanent damage to the computer. In either case, productivitydecreases and cost increases. In addition, in some computingapplications, such as computer servers, the computers must be runcontinuously, regardless of whether there is a fan failure.

A solution to this problem would be to install multiple fans to servethe computer. In this age of smaller and smaller computers, however,space is at a premium, and every fan added means larger computers,higher costs, and less space for other components. Fans installed in aparallel configuration take up significantly more room than a single fanalone, and the installation of the second fan may not be in a locationthat allows for optimal airflow over the computer components.

Fans installed in series suffer from different problems. If one fan isnot functioning, the other fan would be operating less efficiently,trying to move air past the non-operational fan. If the non-operationalfan is in a free-wheeling condition (i.e., spinning due to airflow overthe blades), efficiency of the operational fan is still reduced. Alocked-rotor condition in the non-operational fan poses an even greaterobstacle to proper airflow. These conditions also may lead to reducedfan life for the operational fan. The solution to this problem would beto remove the non-operational fan from the path of the airflow, but thissolution is not practical.

Another drawback associated with current tube axial fans is that thenumber of blades that can be formed on a given fan is limited by theconstruction of, and processes used to manufacture such fans. Typically,the greater the number of blades that can be formed on a tube axial fanof given size and geometry, the greater will be the flow rate of the fanat each pressure point (i.e., the pressure differential between theinlet and outlet of the fan during operation). Accordingly, in order toincrease the flow rate of a tube axial fan of given size and geometry,it is desirable to maximize the number of blades on the fan. In certainapplications, this may involve forming the blades in an overlappingmanner (i.e., wherein the trailing edge of each blade angularly overlapsthe leading edge of the adjacent blade). In many commercially availabletube axial fans, the blades and rotor hub are formed of plastic andinjection molded as a single part. Typically, the molding equipmentrequires that the adjacent edges of the blades be angularly spacedrelative to each other (i.e., the trailing edge of each blade must beangularly spaced from the leading edge of the next blade). Otherwise,the manufacturing process would require overly complex and/or expensivemolding equipment that would render the fan commercially impractical.Accordingly, it has been considered impractical to injection mold rotorhub and blade assemblies with overlapping blades. One manufacturer hasprovided a tube axial fan with overlapping blades by welding the bladesto the rotor hub. However, this manufacturing process is believed to berelatively expensive and therefore undesirable in comparison to theprocesses used to injection mold conventional tube axial rotor hub andblade assemblies.

What is needed then, is a collapsible fan and fan system that provideredundant airflow to a computer or other electronic applications (e.g.,communication, appliance and automotive applications) without taking upmuch more space than a single fan. Also, to reduce fan inefficienciesthat occur when an operational fan attempts to move air past anon-operational fan, a collapsible fan and redundant fan system thatminimize blockage by the non-operational fan would be desired.Collapsible fans already exist in applications other than computers orelectronic applications wherein the fans fold during periods ofnon-operation, using a mechanical means (e.g., a spring), the dynamicpressure of the passive flow of fluid over the blades, or gravity. Someof these fans are forced open due to centrifugal motion when operating.None of these fans, however, address the problem of reduced fanefficiency in series mounted fans in the event of a fan failure.Similarly, none of the existing collapsible fans address the problem ofcontinuously operating a computer or other electronic apparatus (e.g.,communications equipment, appliances, and automotive equipment) in theevent of a fan failure.

Accordingly, it is an object of the present invention to overcome one ormore the above-described drawbacks and disadvantages of the prior art.

SUMMARY OF THE INVENTION

One aspect of the present invention is directed to a fan havingcollapsible fan blades and a system and method employing the collapsiblefan for providing redundant airflow in a manner that reduces the faninefficiency caused when one fan in a series mounted pair is notoperating, either because it is free-wheeling or in a locked rotorcondition. The first or collapsible fan includes collapsible blades thatopen due to centrifugal motion when the fan is operating. If and whenthe first fan ceases to operate, either because of mechanical orelectrical failure or otherwise, the second fan can be energized.Energizing the second fan creates an airflow that forces the collapsibleblades of the first fan to fold inward and prevent undesirable blockageof the air stream by the blades of the non-operational fan. Inaccordance with another aspect of the present invention, the collapsibleblades angularly overlap one another such that the leading edge of oneblade angularly overlaps the trailing edge of an adjacent blade.

One advantage of the currently preferred embodiments of the presentinvention is that they provide a collapsible fan and redundant fansystem and method for computer systems and other electronicapplications, thereby eliminating cost and downtime in the event of afan failure. Another advantage is that the collapsible fan and redundantfan system can be installed in less space than would be possible if twofans were installed in parallel. Still another advantage is thatefficiency of the operating fan will be less affected if the blades ofthe non-operational fan are effectively removed from the air stream,thus maintaining cooling efficiency and reducing additional wear on theoperational fan. Yet another advantage of a preferred embodiment of thepresent invention is that the blades may angularly overlap one another,thereby enabling more blades to be mounted on the hub in comparison toprior art tube axial fans of the same size, and thus enabling increasedflow rates at given pressure points in comparison to such prior artfans.

These and other features and advantages of the present invention aremore fully disclosed or rendered apparent from the following detaileddescription of certain preferred embodiments of the invention, that areto be considered together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cut-away, perspective view of a computer employing acollapsible fan and redundant fan system of the present invention;

FIG. 2A is a front view of the fan with collapsible blades of FIG. 1 inthe open position;

FIG. 2B is a perspective view of the fan with collapsible blades of FIG.1 in the open position;

FIG. 3A is a front view of the fan with collapsible blades of FIG. 1 inthe closed or collapsed position;

FIG. 3B is a perspective view of the fan with collapsible blades of FIG.1 in the closed or collapsed position;

FIG. 4A is a partial, perspective view of one of the fans of FIGS. 2 and3 illustrating the pivot mounts of a pivot connector formed on the fanrotor;

FIG. 4B is a perspective view of a collapsible fan blade of one of thefans of FIGS. 2 and 3 illustrating the pivot pins that are receivedwithin the pivot mounts of FIG. 4A for pivotally mounting thecollapsible blades to the rotor;

FIG. 5A is a partial, perspective view of one of the fans of FIGS. 2 and3 illustrating the pivot mounts formed on the fan rotor of anotherembodiment of a pivot connector;

FIG. 5B is a perspective view of a collapsible fan blade of one of thefans of FIGS. 2 and 3 illustrating in further detail the base portion ofthe fan blade;

FIG. 5C is a partial, side elevational, exploded view of the fan of FIG.5A illustrating the assembly of a pivot pin to a collapsible blade;

FIG. 5D is another partial, side elevational, exploded view of the fanof FIG. 5A illustrating the assembly of the pivot pin and collapsibleblade to the rotor;

FIG. 5E is an enlarged, elevational view of a portion of the collapsiblefan blade of FIG. 5D showing the pin seated against the stop surface ofthe blade;

FIG. 6A is a front elevational view of another fan embodying the presentinvention including collapsible blades that angularly overlap oneanother (i.e., the leading edge of each blade angularly overlaps thetrailing edge of the adjacent blade) and illustrating the blades in anopen condition;

FIG. 6B is a rear elevational view of the fan of FIG. 6A illustratingthe blades in a collapsed or closed condition;

FIG. 7A is a side elevational view of the fan of FIG. 6A illustratingthe blades in an open condition; and

FIG. 7B is a side elevational view of the fan of FIG. 6A illustratingthe blades in a collapsed or closed condition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The currently preferred embodiments of the present invention overcomemany of the problems that arise when fan systems that are used incomputers and other electronic applications fail. The advantages, andother features of the disclosed fan, system and method, will become morereadily apparent to those having ordinary skill in the pertinent artfrom the following detailed description of certain preferred embodimentstaken in conjunction with the drawings which set forth representativeembodiments of the present invention and wherein like reference numeralsidentify similar structural elements.

In FIG. 1, numeral 10 generally refers to a computer having a computerhousing 12 served by a redundant fan system. The computer housing 12defines at least one opening 14 and contains a number of computercomponents 16 (e.g., transformers, processors, circuitry, integratedcircuits, power supplies, or like elements). A redundant fan system ofthe present invention includes a first fan 18 a and a second fan 18 b.As can be seen, the second fan 18 b is axially aligned with the firstfan 18 a such that the two fans are mounted in series. In thisembodiment, both the first fan 18 a and the second fan 18 b of theredundant fan system have collapsible blades. However, as describedfurther below, only one of the fans may have collapsible blades if sodesired.

The opening 14 of the computer housing 12 may be covered with a mesh,screen, or other means for preventing entry of foreign objects inthrough the opening 14. For clarity, the computer components 16 areshown in FIG. 1 as being installed in line with the fans 18 a- 18 b. Oneof ordinary skill in the pertinent art will recognize that FIG. 1 is asimplified configuration of a computer. The redundant fan system of thepresent invention could also be installed in computers that have morethan a single opening, and the exact location of computer components isnot critical to the disclosure of the present invention. It isdesirable, though, that the fan system be located so as to allow themost airflow possible over the various components and otherwise to mostefficiently cool the components.

Still referring to FIG. 1, the first fan 18 a and the second fan 18 bare installed axially along the center of each fan's rotor. A person ofordinary skill in the pertinent art will recognize that the exactlocation and spacing between the first fan 18 a and the second fan 18 bcan vary depending on space restraints or other considerations. Forexample, the fans 18 a- 18 b may be installed with housings in contactor may be separated by a predetermined distance. In the illustratedembodiment, spacing between the first fan 18 a and the second fan 18 bmay be set such that substantially laminar airflow is achieved beforethe air stream contacts the non-operational fan. This would lessen thepossibility of the collapsible blades of the non-operational fan movingor chattering if very turbulent air were to contact the collapsedblades. A person of ordinary skill in the pertinent art will alsorecognize that the first fan 18 a and the second fan 18 b can beinstalled to move air either into or out of the computer housing.

In the embodiment of FIG. 1, both the first fan 18 a and second fan 18 bare shown as having collapsible blades. A person of ordinary skill inthe pertinent art will readily appreciate that the first fan 18 a couldhave fixed blades and the second fan 18 b could have collapsible blades,or vice versa. This type of system, where one fan has collapsible bladesand the other has fixed blades, may reduce total cost in comparison to asystem where both fans have collapsible blades.

FIG. 1 shows a redundant fan system installed in a computer housing 12,but the fan, system and method of the present invention are not limitedto applications related only to computers. The fan, system and method ofthe present invention would be equally useful in applications where fanredundancy is required or desired. For example, the fan, system andmethod of the present invention could be used in communicationsequipment, appliances, automotive equipment, building ventilationsystems, fume exhaust systems, or in other air moving systems. Inaddition, the fan, system and method of present invention could beemployed in any series mounted fan application where minimal effect onthe operational fan is desired.

A person of ordinary skill in the pertinent art also will recognizethat, in computer applications, both fans may be installed not justwithin the enclosure itself. Rather, one or both fans may be mounted tothe outside of the enclosure on a frame or similar supporting device. Ina larger computer, for example, mounting a collapsible blade fan on theexterior of a housing that contains only a single fan would create anafter-market redundant fan system. Mounting at least one fan exterior tothe housing also can simplify or otherwise facilitate the ability torepair or replace the fan. In that example, if the primary fan wereinstalled on the exterior of the enclosure and were to fail, thesecondary fan would be energized, collapsing the blades of the primaryfan. Once support personnel notice the failed primary fan, they couldeasily remove the primary fan, even while the secondary fan isoperating, and replace it. Once replaced, the secondary fan could bede-energized and the new primary fan energized, collapsing the blades ofthe secondary fan.

Various types of control systems are contemplated for the redundant fansystem of the present invention. As shown in broken lines in FIG. 1, theredundant fan system includes a controller 17 electronically coupled tothe first and second fans 18 a and 18 b, respectively. As may berecognized by those of ordinary skill in the pertinent art based on theteachings herein, virtually any type of controller presently availableor developed in the future may be used to control the two fans. Also,the control system preferably includes a sensor 19 for sensing a failureof an operational fan that is electronically coupled to the controller17 and to each fan having collapsible blades. The sensor 19 may take theform of any of numerous structures that are currently or later becomeknown for performing the function of sensing a failure of an operationalfan, including devices that measure or sense airflow, pressure changes,temperature changes within the computer housing, electrical currentflows, or other conditions that would indicate that a fan has failed,such as a vane switch, a locked rotor signal, a pressure sensor or atemperature gauge.

Referring now to FIGS. 2A, 2B, 3A, and 3B, numeral 18 generally refersto a fan with collapsible blades to be used in a redundant fan system.In FIGS. 2A and 2B, the collapsible blades 28 are shown in the openposition. In FIGS. 3A and 3B, the collapsible blades 28 are shown in thecollapsed position. The fan frame 20 has a first aperture 22 and asecond aperture 24 located on the opposite side of the fan frame 20. Thefan frame 20 contains a fan rotor 26 defining a rotor hub and aplurality of collapsible blades 28 pivotally connected to the rotor hubat pivot connectors 30. The fan rotor 26 includes mounted within therotor hub an electric motor and wiring (not shown). In the currentlypreferred embodiments of the present invention, the motor is a brushlessdc motor; however, as may be recognized by those of ordinary skill inthe pertinent art based on the teachings herein, the motor may take anyof numerous different configurations that are currently or later becomeknown.

In the preferred embodiments, the fan frame 20 is the same general sizeand configuration as any of numerous different fans available in theprior art. A fan frame 20 that is the same size as existing computerfans is desired, but since the redundant fan system of the presentinvention is usable in other applications, other fan sizes and/orconfigurations can be employed as needed. In the illustrated embodiment,the first aperture 22 defines an inlet and the second aperture 24defines an outlet. A person of ordinary skill in the pertinent art willrecognize that the location of the inlet 22 and the outlet 24 depends onthe airflow direction (i.e., the inlet 22 is not necessarily the freeend of the rotor 26, as indicated in the Figures). In the preferredembodiment, the inlet 22 and the outlet 24 are unobstructed. If,however, the fan 18 is installed in a location where contact with movingblades is possible, a mesh or screen may be provided on either the inlet22, the outlet 24, or both, to reduce the possibility of contact withthe blades and to prevent intrusion of foreign objects into the fan.

The fan 18 contains a rotor 26 with a plurality of pivot connectors 30secured to the exterior of the rotor 26. The exact number of pivotconnectors 30 can vary depending on space, cost, or application that theredundant fan system is used for. Similarly, the orientation of thepivot connectors 30 with respect to the fan rotor 26 may vary providedthe blades 28 collapse substantially in the direction of the airflowthrough the fan frame 20 when the fan 18 is not operating. In acurrently preferred embodiment, the pivot connectors 30 are installed atan angle oblique to the axis of the rotor 26. Snap-fit or pin-type pivotconnectors 30 are currently contemplated; however, as may be recognizedby those of ordinary skill in the pertinent art based on the teachingsherein, the pivot connectors may take any of numerous differentconfigurations that are currently or later become know for performingthe function of the connectors disclosed herein.

As shown in FIGS. 4A and 4B, a typical snap-fit pivot connector 30includes a pair of first pin mounts 32 located on the peripheral surfaceof the rotor 26 and a second pin mount 34 located on the peripheralsurface of the rotor and spaced between the first pin mounts 32. Asshown in FIG. 4A, each of the first and second pin mounts 32 and 34,respectively, defines an axially-elongated groove or recess 36. As shownin FIG. 4B, each collapsible blade 28 includes on its base three pivotpins 38 with flange portions 40 formed at the ends of the pivot pins.The pins 38 are received within the grooves 36 of the respective pinmounts 32, 34 to thereby form a respective pivot connector 30. As can beseen, the grooves 36 and corresponding pins 38 are shaped anddimensioned to form a snap-fit such that the pins are retained withinthe grooves, but are allowed to pivot within the grooves to therebypivotally mount the collapsible blades to the rotor. As shown typicallyin FIG. 4A, the grooves 36 of each pivot connector 30 are axiallyaligned with each other, and the axes of the grooves are oriented at anoblique angle with respect to the axis of the rotor to thereby mount thecollapsible blades on the rotor at the oblique angle. As also showntypically in FIG. 4A, each second pin mount 34 defines a radiallyextending post 42 that engages the adjacent surface of the respectivecollapsible blade 28 when the fan is rotating to prevent further outwardmovement of the blade and thereby fix the position of the blade in theopen or operational position.

In FIGS. 5A through 5E, another pivot connector employed in the fansystem of the present invention is indicated generally by the referencenumeral 130. The pivot connector 130 is substantially similar to thepivot connector 30 described above, and therefore like referencenumerals preceded by the numeral “1” are used to indicate like elements.A primary difference of the pivot connector 130 in comparison to thepivot connector 30 is that the connector 130 includes a single pivot pin138 that is slidably received through axially-elongated apertures 140formed through the flange portions 140 of the respective collapsibleblade 128. As shown typically in FIG. 5B, the base of each collapsibleblade defines a stop surface 146 for engaging one end of the respectivepivot pin 138. In order to assemble each pivot connector 130, the pivotpin 138 is slidably inserted through the apertures 144 in the flangeportions 140 until the end of the pin engages the stop surface 146, asshown typically in FIG. 5E. Then, the assembled blade 128 and pivot pin138 are moved into engagement with the respective mounts 132, 134 on therotor 126 until the pivot pin 138 is snapped into the grooves 136 of themounts to thereby retain the pin within the mounts. As shown typicallyin FIG. 5A, one of the first mounts 132 includes an end wall 148 forpreventing axial movement of the pin and thus retaining the pin withinthe mounts.

The collapsible blades 28 may be constructed in any shape or sizerequired for the particular application in which the redundant fansystem is used. In the preferred embodiment shown, the collapsibleblades 28 are generally square shaped with rounded outer edges. Otherblade shapes can be used depending on the specific application,including, but not limited to, oval, teardrop, square, oblong, or anyother shape now known in the art or developed in the future.

In FIGS. 6A through 7B, another tube axial fan with collapsible bladesembodying the present invention is indicated generally by the referencenumber 218. The fan 218 is substantially similar to each fan 18described above, and therefore like reference numerals preceded by thenumeral “2”, or preceded by the numeral “2” instead of the numeral “1”,are used to indicate like elements. The primary difference of the fan218 in comparison to the fans 18 described above is that the blades 228angularly overlap one another. As shown typically in FIG. 6A, each blade228 defines a leading edge 227 and a trailing edge 228. As can be seen,each leading edge 227 angularly overlaps the trailing edge 229 of theadjacent blade by an angular amount “x”. Accordingly, a significantadvantage of the fan 218 is that the number of blades 228 for a givenfan size can be increased in comparison to the number of blades mountedon a fan of the same size wherein the blades and rotor hub aremanufactured by conventional injection molding without collapsibleblades. As may be recognized by those of ordinary skill in the pertinentart based on the teachings herein, the degree of angular overlap, shapesand dimensions of the overlapping blades 228 may be set as desired orotherwise required in order to enhance the performance of the fan and/orotherwise meet the objectives a particular application. As can be seenin FIGS. 7A and 7B, the fan 218 includes a shaft 227 mounted on the axisof the rotor 226 for rotatably supporting the fan on a fan frame (notshown). In addition, the pivot connectors 230 may be the same as thepivot connectors described above. Alternatively, as shown typically inFIG. 7A, the post 242 for engaging the respective blade in the openposition may extend laterally adjacent to the base of the blade.However, as indicated above, the pivot connectors may take any ofnumerous different configurations that are currently or later becomeknown for performing their function as disclosed herein.

While standard computer fans are constructed of plastic, the componentsof the fan and redundant fan system of the present invention could beconstructed of plastic, aluminum, steel, or other materials, or anycombination of such materials. Material selection may vary based on theapplication that the fan or redundant fan system is used in, and on anyproperties that the fan components must have (e.g., corrosionresistance, heat resistance, etc.). The fans and redundant fan system ofthe present disclosure may operate effectively regardless of materialselected for the fan housing. It is currently desired, however, that thecollapsible blades be of a material that is light enough to fold inwardwhen in the non-operational state and when air from the operational fanis moving over the collapsible blades.

Although the invention has been described in terms of exemplaryembodiments, it is not limited thereto. Rather, the appended claimsshould be construed broadly, to include other variants and embodimentsof the invention, which may be made by those skilled in the art withoutdeparting from the scope and range of equivalents of the invention asdefined by the appended claims.

1. A collapsible fan for use in a redundant fan system comprising afirst fan including a fan frame, a rotor rotatably mounted on the fanframe, a first aperture located on one side of the rotor, a secondaperture located on an opposite side of the rotor relative to the firstaperture, and a plurality of fan blades mounted on the rotor, thecollapsible fan comprising: a fan frame, a rotor rotatably mounted onthe fan frame, a first aperture located on one side of the rotor, asecond aperture located on an opposite side of the rotor relative to thefirst aperture, and a plurality of collapsible fan blades pivotallymounted on the rotor, wherein the collapsible fan is mountable adjacentto one of the first and second apertures of the first fan, and thecollapsible fan blades are collapsible onto the respective rotor inconnection with an airflow generated by the first fan.
 2. Thecollapsible fan set forth in claim 1, further comprising a sensor forsensing a failure of the collapsible fan.
 3. The collapsible fan setforth in claim 2, further comprising: a controller coupled to the sensorfor energizing the first fan in response to a failure of the collapsiblefan to, in turn, collapse the plurality of collapsible blades.
 4. Thecollapsible fan set forth in claim 2, wherein the sensor is selectedfrom the group including a vane switch, a locked rotor signal, apressure sensor, and a temperature gauge.
 5. The collapsible fan setforth in claim 1, wherein the plurality of collapsible blades are openedand maintained in the open position in response to centrifugal forcegenerated by rotation of the rotor.
 6. The collapsible fan set forth inclaim 1, further comprising a plurality of pivot connectors thatpivotally connect a plurality of collapsible blades to the rotor.
 7. Thecollapsible fan set forth in claim 6, wherein the rotor includes aplurality of pivot mounts angularly spaced relative to each other, andeach pivot connector includes a pivot pin received within the respectivepivot mount for pivotally connecting a respective blade to the rotor. 8.The collapsible fan set forth in claim 1, wherein the fan is a brushlessdc fan.
 9. The collapsible fan set forth in claim 1, wherein a pluralityof the collapsible fan blades angularly overlap one another.
 10. Thecollapsible fan of claim 1, in combination with a redundant fan system,comprising: a first fan including a fan frame, a rotor rotatably mountedon the fan frame, a first aperture located on one side of the rotor, asecond aperture located on an opposite side of the rotor relative to thefirst aperture, and a plurality of fan blades mounted on the rotor; andwherein the collapsible fan forms a second fan mounted adjacent to oneof the first and second apertures of the first fan, and the collapsiblefan blades of the second fan are collapsible onto the respective rotorupon rotating the first fan rotor.
 11. The redundant fan system setforth in claim 10, further comprising a frame for mounting the first fanand second fan.
 12. The redundant fan system set forth in claim 10,further comprising: an enclosure having at least one outer wall, theenclosure defining an interior and an opening in the at least one outerwall, and wherein the first fan is located within the enclosure adjacentto the opening.
 13. The redundant fan system set forth in claim 12,wherein the enclosure is a computer housing.
 14. The redundant fansystem set forth in claim 10, wherein both the first fan blades andsecond fan blades are collapsible.
 15. The collapsible fan set forth inclaim 1, wherein the plurality of collapsible blades are maintained inan open position in response to centrifugal force generated by rotationof the collapsible fan rotor, and collapse due to an airflow of thefirst fan.
 16. A collapsible fan for use in a redundant fan system,wherein the redundant fan system comprises the collapsible fan and afirst fan including a fan frame, a rotor rotatably mounted on the fanframe, a first aperture located on one side of the rotor, a secondaperture located on an opposite side of the rotor relative to the firstaperture, and a plurality of fan blades mounted on the rotor, thecollapsible fan comprising: a fan including a fan frame, a rotorrotatably mounted on the fan frame, a first aperture located on one sideof the rotor, a second aperture located on an opposite side of the rotorrelative to the first aperture, and a plurality of fan blades mounted onthe rotor; and means for collapsing the fan blades toward the rotor inconnection with an airflow generated by the first fan.
 17. Thecollapsible fan set forth in claim 16, wherein a plurality of thecollapsible fan blades angularly overlap one another.
 18. Thecollapsible fan set forth in claim 16, wherein the means for collapsingis defined by a plurality of pivot connectors, and each pivot connectorpivotally connects a respective fan blade to the rotor.
 19. Thecollapsible fan set forth in claim 18, wherein each pivot connectorincludes at least one mounting surface on one of the rotor and theblade, and at least one pivot pin on the other of the rotor and theblade that is retained on the mounting surface.
 20. The collapsible fanset forth in claim 16, further comprising: first means for energizingthe first fan in response to a failure of the collapsible fan to, inturn, collapse the plurality of collapsible blades of the failedcollapsible fan; and second means coupled to the first means for sensinga failure of the collapsible fan.
 21. The collapsible fan set forth inclaim 20, wherein the second means is selected from the group includinga vane switch, a locked rotor signal, a pressure sensor, and atemperature gauge.
 22. The collapsible fan of claim 16 in combinationwith a redundant fan system, comprising: a first fan including a fanframe, a rotor rotatably mounted on the fan frame, a first aperturelocated on one side of the rotor, a second aperture located on anopposite side of the rotor relative to the first aperture, and aplurality of fan blades mounted on the rotor.
 23. The redundant fansystem set forth in claim 22, in combination with a computer defining ahousing and a plurality of computer components mounted within thehousing, and wherein the redundant fan system is mounted on the housingfor cooling the components within the housing.
 24. The redundant fansystem set forth in claim 22, wherein the two fans are substantiallyaxially aligned with each other.
 25. A collapsible fan for use in aredundant fan system comprising a first fan including a fan frame, arotor rotatably mounted on the fan frame, a first aperture located onone side of the rotor, a second aperture located on an opposite side ofthe rotor relative to the first aperture, and a plurality of fan bladesmounted on the rotor, the collapsible fan comprising: a fan frame, arotor rotatably mounted on the fan frame, a first aperture located onone side of the rotor, a second aperture located on an opposite side ofthe rotor relative to the first aperture, and a plurality of collapsiblefan blades pivotally mounted on the rotor at an angle oblique to an axisof the rotor, wherein the collapsible fan is mountable adjacent to oneof the first and second apertures of the first fan, and the collapsiblefan blades are collapsible onto the respective rotor upon rotating thefirst fan rotor.
 26. A collapsible fan for use in a redundant fan systemcomprising a first fan including a fan frame, a rotor rotatably mountedon the fan frame, a first aperture located on one side of the rotor, asecond aperture located on an opposite side of the rotor relative to thefirst aperture, and a plurality of fan blades mounted on the rotor, thecollapsible fan comprising: a fan frame, a rotor rotatably mounted onthe fan frame, a first aperture located on one side of the rotor, asecond aperture located on an opposite side of the rotor relative to thefirst aperture, a plurality of collapsible fan blades pivotally mountedon the rotor, a plurality of pivot connectors that pivotally connect aplurality of collapsible blades to the rotor, and a plurality of pivotmounts angularly spaced relative to each other about the rotor, whereineach pivot connector includes a pivot pin received within the respectivepivot mount for pivotally connecting a respective blade to the rotor,each pivot mount includes a recess, and each pivot pin is frictionallyengaged within the respective recess to secure the respective blade tothe blade mount, and wherein the collapsible fan is mountable adjacentto one of the first and second apertures of the first fan, and thecollapsible fan blades are collapsible onto the respective rotor uponrotating the first fan.
 27. A method of providing redundant airflowcomprising the steps of: providing a first fan including a plurality ofcollapsible fan blades; providing a second fan adjacent to the firstfan; and activating the second fan upon a failure of the first fan andcollapsing the blades of the first fan in connection with an airflowgenerated by the second fan.
 28. The method set forth in claim 27,further comprising the step of sensing the failure of the first fan, andactivating the second fan upon sensing the failure of the first fan. 29.The method set forth in claim 27, further comprising the step ofmounting the first and second fans in approximate axial alignment witheach other.
 30. The method set forth in claim 27, further comprising thestep of mounting at least one of the first and second fans within acomputer housing.
 31. A collapsible fan for use in a redundant fansystem comprising a first fan including a fan frame, a rotor rotatablymounted on the fan frame, a first aperture located on one side of therotor, a second aperture located on an opposite side of the rotorrelative to the first aperture, and a plurality of fan blades mounted onthe rotor, the collapsible fan comprising: a fan including a fan frame,a rotor rotatably mounted on the fan frame, a first aperture located onone side of the rotor, a second aperture located on an opposite side ofthe rotor relative to the first aperture, and a plurality of fan bladesconnected to the rotor at an angle oblique to an axis of the rotor; andmeans for collapsing the fan blades toward the rotor upon rotating therotor of the first fan.
 32. The collapsible fan set forth in claim 31,wherein a plurality of the collapsible fan blades angularly overlap oneanother.
 33. The collapsible fan set forth in claim 31, wherein themeans for collapsing is defined by a plurality of pivot connectors, andeach pivot connector pivotally connects a respective fan blade to therotor at an angle oblique to the axis of the rotor.
 34. The collapsiblefan set forth in claim 33, wherein each pivot connector includes atleast one mounting surface on one of the rotor and the blade, and atleast one pivot pin on the other of the rotor and the blade that isretained on the mounting surface.
 35. A collapsible fan as defined inclaim 31, wherein said means collapses the fan blades in connection withan airflow generated by the first fan.